1. Field of the Invention
The present invention generally relates to an interchangeable lens camera system, and more particularly relates to a camera system in which an intermediate accessory, such as an intermediate ring, a bellows, or the like, which is used for close-up photographing, can be mounted or interposed between an interchangeable objective lens and a camera body, and also relates to an interchangeable lens, a close-up photographic intermediate accessory, and a camera body, which are used in such a camera system.
2. Description of the Prior Art
Conventionally, when an intermediate accessory, such as a tele-converter or the like, is mounted between a camera body and an interchangeable objective lens, it is known that vignetting is caused within the intermediate accessory. Such a technique whereby the amount of vignetting occurring in the intermediate accessory is transmitted from the intermediate accessory to the camera body so as to correct or restrict exposure control, has been proposed, for example, in U.S. Pat. No. 4,326,788.
The present invention recognizes the fact that the foregoing phenomenon of the vignetting is caused in an interchangeable objective lens in the state where the close-up photographic intermediate accessory is used. The phenomenon of such vignetting is described hereinafter.
In the case where an intermediate accessory is mounted between an interchangeable objective lens and a camera body in order to perform close-up photographing or high magnification photographing, the shortest object distance (which is a distance from an object to be photographed to the object side surface of the interchangeable lens) is shortened in comparison with the case of using only the interchangeable objective lens. Here, the minimum effective F-number of a photographing optical system is determined in accordance with an object distance, an effective diameter of a constituent component of the optical system, and the largest aperture of a diaphragm (or alternatively, an aperture diameter of an F-number determining ring). Referring to FIG. 12, this state will be explained with a thin single lens corresponding to an interchangeable objective lens of the whole shifting type by way of example.
In FIG. 12, the reference symbol L designates a lens; the reference symbol F.sub..infin. designates a film surface in the infinity focusing condition; and the reference symbol F.sub.N designates a film surface in the nearest focusing condition. In the infinity focusing condition, a parallel light bundle having a width the same as an effective aperture D of the lens L is entered as shown by a solid line in FIG. 12 and focused on the film surface F.sub..infin.. At this time, the minimum effective F-number F.sub.No.eff..infin. is determined in accordance with the following equation (1): ##EQU1## where the reference symbol .theta.1 designates an exit angle of outer edge light ray.
In the nearest focusing condition, on the contrary, as shown by a dotted line in FIG. 12, the outer edge light ray emitted from an object O enters the lens L as a divergent light bundle and is focused on the film surface F.sub.N disposed optically behind the film surface F.sub..infin. in the infinity focusing condition. At this time, the minimum effective F-number F.sub.No.eff.N is determined in accordance with an equation of ##EQU2## where the reference symbol .theta..sub.2 designates an exit angle of the outer edge light ray. Since the effective aperture of the thin lens L is equal to the diaphragm aperture in a practical lens system, exposure can be correctly controlled by controlling the diaphragm aperture in the lens system.
In FIG. 12, it is found that in the range from the lens L to the film surface, the outer edge light ray indicated by a dotted line in the nearest focusing condition passes outside (a farther path from an optical axis) the outer edge light ray indicated by a solid line in the infinity focusing condition. That is, this fact shows that in the thick lens system of the whole shifting type, the outer edge light ray in the nearest focusing condition passes outside the outer edge light ray in the infinity focusing condition, in the range behind a diaphragm position, that is, in the range from the diaphragm position to the film surface. Here, ordinarily, an effective diameter of each lens surface of the lens system is determined so that even in the nearest focusing condition of the lens itself, the outer edge light ray is not interrupted. Therefore, the width of the light bundle used in photographing is always controlled by the diaphragm. If the outer edge light ray is interrupted by the effective diameter of the lens disposed behind the diaphragm in the nearest focusing condition, the exit angle .theta..sub.2 is determined by the effective diameter of the lens, so that the outer edge light ray passes inside (a path closer to an optical axis) the diaphragm aperture at the diaphragm position. Therefore, even when the aperture of diaphragm is started to be decreased from the maximum state, there occurs such a phenomenon that the width of the light bundle is not changed. This phenomenon is referred to as "a phenomenon of partial light blocking aperture". In an ordinary interchangeable objective lens, the effective diameter of each lens surface is determined so that such a phenomenon of partial light blocking aperture is not caused in the whole range of focusing distance of the interchangeable objective lens.
Now, in a camera system of the full aperture light measuring type, as shown in FIG. 13, let an AV value corresponding to the minimum effective F-number be represented by AV.sub.0 ; an AV value corresponding to the desired diaphragm aperture be represented by AV.sub.1 ; and an AV value corresponding to a difference between the width of the outer edge light ray and the diaphragm aperture due to the phenomenon of partial light blocking aperture be represented by .DELTA.AV.sub.0 ; then a diaphragming stroke .DELTA.AV.sub.1 to obtain the desired diaphragm aperture AV.sub.1 is is expressed by the following equation (3): EQU .DELTA.AV.sub.1 =AV.sub.0 +(AV.sub.1 -AV.sub.0) (3)
Here, the .DELTA.AV.sub.0 is referred to as the "amount of partial light blocking aperture". In the ordinary interchangeable objective lens, the effective diameter of each lens surface is determined so that the amount of partial light blocking aperture .DELTA.AV.sub.0 is a negligibly small value in the whole range of focusing distance thereof and therefore the diaphragming stroke .DELTA.AV.sub.1 is controlled on the basis of the following equation: EQU .DELTA.AV.sub.1 =AV.sub.1 -AV.sub.0 ( 4)
In the case where the close-up photographic intermediate accessory is mounted between the interchangeable objective lens and the camera body, however, the shortest object distance is considerably shortened in comparison with the case of using only the interchangeable objective lens. This state will be now described with reference to FIGS. 14 and 15.
FIG. 14 shows the case where an intermediate accessory having no lens system, such as an intermediate ring, a bellows, or the like, is used. In the drawing, a solid line indicates the outer edge light bundle in the infinity focusing condition in the state where the intermediate accessory is used. The reference symbol F.sub.N1 designates a film surface in the case of using the intermediate accessory. FIG. 15 shows an intermediate accessory C having a lens system, such as a macro-converter, or the like. In the drawing, a solid line indicates the outer edge light bundle in the infinity focusing condition in the case where no intermediate accessory is used, while a dotted line indicates the outer edge light bundle in the nearest focusing condition in the case where an intermediate accessory is used. The reference symbol F.sub.N2 designates a film surface in the case of using the intermediate accessory.
As will be apparent from FIGS. 14 and 15, the use of an intermediate accessory causes the outer edge light bundle (indicated by the dotted line) in the nearest focusing condition to pass outside the outer edge light bundle (indicated by the solid line) in the infinity focusing condition. In order to prevent such a phenomenon of partial light blocking aperture as described above from occurring even in the state where an intermediate accessory is used, it is necessary that not only the effective diameter of each lens surface of the interchangeable objective lens disposed behind a diaphragm, but also the effective diameter of the intermediate accessory are made large. The ordinary interchangeable objective lens, however, is designed so that no phenomenon of vignetting is caused merely in the case of using only the objective lens without taking the case of using any intermediate accessory into consideration. This is because if the case of using any intermediate accessory is taken into consideration, the diameter of the lens disposed behind the diaphragm of the interchangeable objective lens becomes extremely large and therefore not only the interchangeable objective lens becomes heavy, large, expensive, but also the aberration in off-axial region is determined.
The same applies also to the intermediate accessory, and if it is considered to prevent the phenomenon of partial light blocking aperture from occurring with respect to any interchangeable objective lens, the intermediate accessory per se becomes large and heavy, particulary, in the case where a macroconverter is mounted on the interchangeable objective lens having a large aperture, the light bundle is interrupted in the lens system of the converter, resulting in the phenomenon of the partial light blocking aperture. This is known, for example, in the above-mentioned U.S. Pat. No. 4,326,788.
Conventionally, however, much attention has not been paid on such a phenomenon of vignetting caused by the interruption of the light bundle in the interchangeable objective lens itself because of mounting a close-up photographic intermediate accessory. The present invention has paid attention to such a phenomenon of partial light blocking aperture due to the vignetting caused in an interchangeable lens. Description will be made as to this phenomenon more in detail. FIG. 16 shows the state where an interchangeable objective lens 2 is mounted on camera body 4, and the diameter of the effective light bundle is controlled by the aperture diameter of a diaphragm S in the interchangeable objective lens 2. FIG. 17 shows the state where an intermediate ring 6 is mounted between the interchangeable objective lens 2 and the camera body 4. In this state, the width of the effective light bundle is defined by the rearmost surface RR of a rear lens group RG in the interchangeable objective lens 2 and the diaphragm S does not define the light bundle in the beginning of stopping down of the diaphragm S. That is, the phenomenon of partial light blocking aperture is caused by the lens system in the interchangeable objective lens 2. The reference symbol F designates a film surface. FIG. 18 shows the state where a macro-converter lens 8 is mounted between the interchangeable objective lens 2 and the camera body 4. Even in this state, the light bundle is interrupted by the rearmost surface RR of the interchangeable objective lens 2, resulting in a similar phenomenon of the partial blocking aperture as described above.
The foregoing phenomenon of partial light blocking aperture in the interchangeable objective lens is caused not only in the lens system of the whole lens shifting type but also in any lens system of any type, such as an internal focusing type, a rear focusing type, and so on. In the case of any type other than the whole lens shifting type, the phenomenon of partial light blocking aperture is generated not only in the range behind the diaphragm position but also in the range in front of the diaphragm position. For example, in the case where a bellows is mounted on the interchangeable objective lens of the front lens shifting type, if the bellows is extended so as to shift the whole of the interchangeable objective lens, the vignetting is caused on a lens surface disposed behind a diaphragm, and in this state, if a front lens of the interchangeable objective lens is shifted, the vignetting is caused on a lens surface disposed in front of the disphragm. That is, the vignetting in the interchangeable objective lens is caused on various different lens surfaces depending on the type of lens, type of focusing, or the like.
Since the amount of the partial light blocking aperture due to the vignetting is not negligibly small, there is such a problem that proper exposure can not be obtained under the exposure operation control performed on the basis of the equation (4) in which the amount of partial light blocking aperture is disregarded.